The proteoglycans and glycosaminoglycan chains of rabbit synovium

Synovium friction properties are influenced by proteoglycan content

The synovium plays a crucial role in diarthrodial joint health, and its study has garnered appreciation as synovitis has been linked to osteoarthritis symptoms and progression. Quantitative synovium structure-function data, however, remain sparse. In the present study, we hypothesized that tissue glycosaminoglycan (GAG) content contributes to the low friction properties of the synovium. Bovine and human synovium tribological properties were evaluated using a custom friction testing device in two different cases: (1) proteoglycan depletion to isolate the influence of tissue GAGs in the synovium friction response and (2) interleukin-1 (IL) treatment to observe inflammation-induced structural and functional changes. Following proteoglycan depletion, synovium friction coefficients increased while GAG content decreased. Conversely, synovium explants treated with the proinflammatory cytokine IL exhibited elevated GAG concentrations and decreased friction coefficients. For the first time, a relationship between synovium friction coefficient and GAG concentration is demonstrated. The study of synovium tribology is necessary to fully understand the mechanical environment of the healthy and diseased joint.

Cartilage-targeting dexamethasone prodrugs increase the efficacy of dexamethasone

Intra-articular administration of glucocorticoids such as dexamethasone is a common treatment for osteoarthritic inflammation and pain. Despite its potent anti-inflammatory properties, multiple barriers hinder the drug's effectiveness in the articular space. In particular, the high turnover rate of the synovial fluid and the dense cartilage extracellular matrix (ECM) lead to poor drug penetration into cartilage. In order to increase the infiltration and retention time, two dexamethasone prodrugs were developed. Firstly, dexamethasone was conjugated to polycationic chitosan, which led to deep and sustained infiltration of the drug into full thickness cartilage, due to its strong electrostatic interactions with the high negative fixed charges of the cartilage ECM. Secondly, dexamethasone was conjugated to a collagen type II-binding peptide, WYRGRL, and this prodrug was shown to be retained in the deep zones of cartilage through specific interactions with cartilage-specific collagen type II bundles. In both cases, active dexamethasone was released from the carrier by ester linkage hydrolysis. Complexing dexamethasone with either chitosan or collagen type II-affinity carriers increased its binding and therapeutic efficacy inside cartilage, compared to the free drug. Both dexamethasone conjugates significantly reduced levels of inflammatory markers and slowed the loss of glycosaminoglycans in an ex vivo model. A single dose of a cartilage-targeting dexamethasone prodrug represents a promising alternative to the repetitive glucocorticoid injections needed to compensate for its rapid clearance from the joint cavity.

Changes in synovial fluid biomarker concentrations following arthroscopic surgery in horses with osteochondritis dissecans of the distal intermediate ridge of the tibia

OBJECTIVE

To quantify concentrations of cartilage oligomeric matrix protein (COMP) and fibromodulin in synovial fluid from the tarsocrural joints (TCJs) of horses with osteochondritis dissecans (OCD) of the distal intermediate ridge of the tibia and determine whether concentrations would change following arthroscopic removal of osteochondral fragments.

ANIMALS

115 client-owned horses with OCD of the TCJ and 29 control horses euthanized for unrelated reasons.

PROCEDURES

COMP and fibromodulin concentrations were measured in synovial fluid from the TCJs of the affected horses before and after osteochondral fragments were removed arthroscopically and in synovial fluid from the TCJs of the control horses after euthanasia. Synovial biopsy specimens from the TCJs of affected and control horses were examined histologically for evidence of inflammation.

RESULTS

Synovial fluid COMP and fibromodulin concentrations prior to surgery in horses with OCD were not significantly different from concentrations in control horses. Fibromodulin, but not COMP, concentration in horses with OCD was significantly decreased after surgery, compared with the concentration before surgery. Fibromodulin concentration was significantly correlated with joint effusion score but not with lameness score or results of a flexion test and was correlated with histologic score for number of synoviocytes on the surface of the synovium but not with score for degree of infiltration of inflammatory cells in the synovium. Synovial fluid COMP concentration was not significantly correlated with clinical or histologic findings.

CONCLUSIONS AND CLINICAL RELEVANCE

Results suggested that fibromodulin, but not COMP, could potentially be a biomarker of joint inflammation in horses with OCD of the TCJ.

Hyaluronan and synovial joint: function, distribution and healing

Synovial fluid is a viscous solution found in the cavities of synovial joints. The principal role of synovial fluid is to reduce friction between the articular cartilages of synovial joints during movement. The presence of high molar mass hyaluronan (HA) in this fluid gives it the required viscosity for its function as lubricant solution. Inflammation oxidation stress enhances normal degradation of hyaluronan causing several diseases related to joints. This review describes hyaluronan properties and distribution, applications and its function in synovial joints, with short review for using thiol compounds as antioxidants preventing HA degradations under inflammation conditions.

High affinity glycosaminoglycan and autoantigen interaction explains joint specificity in a mouse model of rheumatoid arthritis

In the K/BxN mouse model of rheumatoid arthritis, autoantibodies specific for glucose-6-phosphate isomerase (GPI) can transfer joint-specific inflammation to most strains of normal mice. Binding of GPI and autoantibody to the joint surface is a prerequisite for joint-specific inflammation. However, how GPI localizes to the joint remains unclear. We show that glycosaminoglycans (GAGs) are the high affinity (83 nm) joint receptors for GPI. The binding affinity and structural differences between mouse paw/ankle GAGs and elbows/knee GAGs correlated with the distal to proximal disease severity in these joints. We found that cartilage surface GPI binding was greatly reduced by either chondroitinase ABC or beta-glucuronidase treatment. We also identified several inhibitors that inhibit both GPI/GAG interaction and GPI enzymatic activities, which suggests that the GPI GAG-binding domain overlaps with the active site of GPI enzyme. Our studies raise the possibility that GAGs are the receptors for other autoantigens involved in joint-specific inflammatory responses.

Interstitial matrix proteins determine hyaluronan reflection and fluid retention in rabbit joints: effect of protease

Hyaluronan (HA) retention inside the synovial cavity of joints serves diverse protective roles. We tested the hypothesis that HA retention is mediated by the network of extracellular matrix proteins in the synovial lining. Cannulated rabbit knee joints were infused with HA solution with or without pretreatment by chymopapain, a collagen-sparing protease. Trans-synovial fluid escape rate was measured and, after a period of trans-synovial filtration, samples of intra-articular fluid and subsynovial fluid were analysed for HA to assess its trans-synovial ultrafiltration. In control joints, HA ultrafiltration was confirmed by postfiltration increases in intra-articular HA concentration (259 +/- 17% of infused concentration) and reduced subsynovial concentration (30 +/- 8%; n = 11). The proportion of HA molecules reflected by the synovium was 57-75%. Chymopapain treatment increased the hydraulic permeability of the synovial lining approximately 13-fold, almost abolished the trans-synovial difference in HA concentration and reduced the HA reflected fraction to 3-7% (n = 6; P < 0.001, ANOVA). Structural studies confirmed that chymopapain treatment depleted the matrix of proteoglycans but preserved its collagen. The findings thus demonstrate that HA ultrafiltration and synovial hydraulic permeability are determined by the network of non-collagen, extracellular matrix proteins. This may be important clinically, since protease activity is raised in rheumatoid arthritis, as are HA and fluid escape.

Ultrastructural localization of hyaluronic acid in the synovium of the goat knee joint

In the Japanese miniature (Shiba) goat, the synovial membrane contains synoviocytes referred to as type A (macrophage-like cells) and type B cells (fibroblast-like cells) in the intimal layer. Small capillaries and blood vessels of varying sizes were located in the extracellular matrix in the synovial subintima. The type A cells in the synovium possessed numerous vesicles, vacuoles and lysosomes as well as pinocytotic vesicles. These ultrastructural features indicating phagocytosis showed distinct positive reactions following hyaluronan staining. On the other hand, in the type B cells, hyaluronic acids were present in the surface coat of the plasma membrane and its periphery. Additionally, perivascular connective tissue of the small capillaries and blood vessels and interfibrous matrix contained hyaluronan. The results suggest that hyaluronic acid, in the synovial tissue, is synthesized on the plasma membrane of type B cells, and taken up by type A cells. Moreover, hyaluronan is involved in cellular functions in the synovial connective tissue.

Ultracytochemical demonstration of glycoproteins in the canine knee synovium

By various ultracytochemical methods, glycoconjugates of the synoviocytes, the intercellular matrix and the wall of the small capillaries were studied in the synovial intimal tissues of the canine knee joint. Glycoconjugates with vicinal diol groups could be visualized in certain elements of the Golgi complex, lysosomes, vacuoles, the majority of intracellular cytomembranes, the surface coat of the plasma membrane and glycogen particles in type A cells. In type B cells, less-developed Golgi complexes, and fewer lysosomes and vacuoles were present in the cytoplasm than in that of type A cells. In contrast, a large number of cytoplasmic glycogen particles and abundant vicinal diol-containing groups in the surface coat of the plasma membrane became especially obvious in the B cells. Abundant neutral and acidic glycoproteins were observed in fibrous components in the intercellular matrix. In the small capillaries, strongly positive staining intensities for neutral and acidic glycoconjugates were observed in the basement membrane and perivascular connective tissue, as well as in the surface coat of the luminal plasma membrane of the endothelial cells, although to a somewhat weaker degree. Sialic acid, particularly, was notable in the surface coat of the latter cells. In addition, glycoproteins in the type A cells were shown by lectin ultracytochemistry to contain a variety of saccharide residues such as alpha-D-mannose, alpha-D-glucose, alpha-L-fucose, N-acetyl-beta-D-glucosamine, and N-acetyl-neuraminic acid, which were also found in the plasma membrane of the B cells. The properties of the glycoconjugates found are discussed in relation to the basic functions assigned to the synovial membrane of the canine knee joint.

Non-linear dependence of interstitial fluid pressure on joint cavity pressure and implications for interstitial resistance in rabbit knee

AIMS

Synovium retains lubricating fluid in the joint cavity. Synovial outflow resistance estimated as dPj/dQs (Pj, joint fluid pressure and Qs trans-synovial flow) is greater, however, than expected from interstitial glycosaminoglycan concentration. This study investigates whether subsynovial fluid pressure increases with intra-articular pressure, as this would reduce the estimated resistance estimate.

METHODS

Interstitial fluid pressure (Pif) was measured as a function of distance from the joint cavity in knees of anaesthetized rabbits, using servo-null pressure-measuring micropipettes and using an external 'window'. Joint fluid pressure Pj was either endogenous (-2.4 +/- 0.4 cmH2O, mean +/- SEM) or held at approximately 4, 8 or 15.0 cmH2O by a continuous intra-articular saline infusion that matched the trans-synovial interstitial drainage rate.

RESULTS

At endogenous Pj the peri-articular Pif was subatmospheric (-1.9 +/- 0.3 cmH2O, n = 19). At raised Pj the Pif values became positive. Gradient dPif /dx was approximately 20 times steeper across synovium than subsynovium. Pif close to the synovium-subsynovium border (Pif*) increased as a non-linear function of Pj to 1.4 +/- 0.2 cmH2O (n = 23) at Pj = 4.3 +/- 0.1 cmH2O : 2.3 +/- 0.2 cmH2O (n = 17) at Pj = 7.6 +/- 0.2 cmH2O: and 3.0 +/- 0.4 cmH2O (n = 26) at Pj = 15 +/- 0.2 cmH2O (P = 0.03, anova).

CONCLUSIONS

Synovial resistivity is approximately 20x subsynovial resistivity. The increase in Pif*with Pj means that true synovial resistance d(Pj-Pif*)/dQs is overestimated 1.5x by dPj/dQs. This narrows but does not eliminate the gap between analysed glycosaminoglycan concentration, 4 mg ml(-1), and the net interstitial biopolymer concentration of 11.5 mg ml(-1) needed to generate the resistance.

Potential mechanism of action of intra-articular hyaluronan therapy in osteoarthritis: are the effects molecular weight dependent?

BACKGROUND

Hyaluronan, or hyaluronic acid (HA), is the major hydrodynamic nonprotein component of joint synovial fluid (SF). Its unique viscoelastic properties confer remarkable shock absorbing and lubricating abilities to SF, while its enormous macromolecular size and hydrophilicity serve to retain fluid in the joint cavity during articulation. HA restricts the entry of large plasma proteins and cells into SF but facilitates solute exchange between the synovial capillaries and cartilage and other joint tissues. In addition, HA can form a pericellular coat around cells, interact with proinflammatory mediators, and bind to cell receptors, such as cluster determinant (CD)44 and receptor for hyaluronate-mediated motility (RHAMM), where it modulates cell proliferation, migration, and gene expression. All these physicochemical and biologic properties of HA have been shown to be molecular weight (MW) dependent.

OBJECTIVE

Intra-articular (IA) HA therapy has been used for the treatment of knee osteoarthritis (OA) for more than 30 years. However, the mechanisms responsible for the reported beneficial clinical effects of this form of treatment remain contentious. Furthermore, there are a variety of pharmaceutic HA preparations of different MW available for the treatment of OA, but the significance of their MWs with respect to their pharmacologic activities have not been reviewed previously. The objective of the present review is to redress this deficiency.

METHODS

We reviewed in vitro and in vivo reports to identify those pharmacologic activities of HA that were considered relevant to the ability of this agent to relieve symptoms and protect joint tissues in OA. Where possible, reports were selected for inclusion when the pharmacologic effects of HA had been studied in relation to its MW. In many studies, only a single HA preparation had been investigated. In these instances, the experimental outcomes reported were compared with similar studies undertaken with HAs of different MWs.

RESULTS

Although in vitro studies have generally indicated that high MW-HA preparations were more biologically active than HAs of lower MW, this finding was not confirmed using animal models of OA. The discrepancy may be partly explained by the enhanced penetration of the lower MW HA preparation through the extracellular matrix of the synovium, thereby maximizing its concentration and facilitating its interaction with target synovial cells. However, there is accumulating experimental evidence to show that the binding of HAs to their cellular receptors is dependent on their molecular size; the smaller HA molecular species often elicits an opposite cellular response to that produced by the higher MW preparations. Studies using large animal models of OA have shown that HAs with MWs within the range of 0.5 x 10(6)-1.0 x 10(6) Da were generally more effective in reducing indices of synovial inflammation and restoring the rheological properties of SF (visco-induction) than HAs with MW > 2.3 x 10(6) Da. These experimental findings were consistent with light and electron microscopic studies of synovial membrane and cartilage biopsy specimens obtained from OA patients administered 5 weekly IA injections of HA of MW = 0.5 x 10(6)-0.73 x 10(6) Da in which evidence of partial restoration of normal joint tissue metabolism was obtained.

CONCLUSIONS

By mitigating the activities of proinflammatory mediators and pain producing neuropeptides released by activated synovial cells, HA may improve the symptoms of OA. In addition, HAs within the MW range of 0.5 x 10(6)-1.0 x 10(6) Da partially restore SF rheological properties and synovial fibroblast metabolism in animal models. These pharmacologic activities of HA could account for the reported long-term clinical benefits of this OA therapy. However, clinical evidence has yet to be described to support the animal studies that indicated that HAs with MW > 2.3 x 10(6) Da may be less effective in restoring SF rheology than HAs of half this size.

Keratan sulfate epitopes exhibit a conserved distribution during joint development that remains undisclosed on the basis of glycosaminoglycan charge density

Changes in glycosaminoglycan (GAG) content and distribution are vital for joint development. However, their precise character has not been established. We have used immunohistochemistry (IHC) and "critical electrolyte" Alcian blue staining to assess such changes in developing chick and rabbit joints. IHC showed chondroitin sulfate labeling in chick epiphyseal cartilage but not in interzones. In contrast, prominent labeling for keratan sulfate (KS) was restricted to chick cartilage-interzone interfaces. In rabbit knees, KS labeling was also prominent at presumptive cavity borders, but weak in interzone and cartilage. Selective pre-digestion produced appropriate loss of label and undersulfated KS was undetectable. Quantification of Alcian blue staining by scanning and integrating microdensitometry showed prominent hyaluronan-like (HA-like) interzone staining, with chondroitin sulfate and weaker KS staining restricted to epiphyseal cartilage. Hyaluronidase decreased HA-like staining in the interzone. Surprisingly, keratanases also reduced HA-like but not sulfated GAG (sGAG-like) staining in the interzone. Chondroitinase ABC had little effect on HA-like staining but decreased sGAG staining in all regions. Rabbit joints also showed HA-like but not KS staining in the interzone and strong chondroitin sulfate-like staining in epiphyseal cartilage. Our findings show restricted KS distribution in the region close to the presumptive joint cavity of developing chick and rabbit joints. Alcian blue staining does not detect this moiety. Therefore, it appears that although histochemistry allows relatively insensitive quantitative assessment of GAGs, IHC increases these detection limits. This is particularly evident for KS, which exhibits immunolabeling patterns in joints from different species that is consistent with a conserved functional role in chondrogenesis.

Proteoglycan production in disomic and trisomy 7-carrying human synovial cells

To gain further insight into the synthesis and structure of the synovial matrix of joints, we have established cell cultures from synovial specimens and elaborated their production of hyaluronan and proteoglycans. The cultures secreted mainly the small proteoglycan decorin, but also considerable amounts of the related biglycan and the large proteoglycan versican. Only minor amounts of heparan sulfate proteoglycans were found. All cultures also had a high production of hyaluronan, which highlights the important role for normal joint function of these cells. In joint diseases, a common feature is the presence of an extra chromosome 7 (trisomy 7) in the synovial cells. To study the possible consequences of trisomy 7 on the synovial cell function, we extended our study to cultures that had been sub-cloned to contain high amounts of trisomy 7-carrying cells. These cell cultures had approximately four times more versican than their disomic counterparts in the cell culture medium, indicating that versican may be a mediator in the processes of joint destructive disorders. To find an explanation for this increase in versican, we investigated the expression/secretion of PDGF-AA and IL-6, cytokines with their genes located to chromosome 7. Indeed, both these cytokines were increased in the cultures with high frequencies of trisomy 7. We then added the two cytokines to cell cultures of disomic synovial cells, but only cells treated with IL-6 displayed an increased amount of versican. Thus, we suggest that the increased amount of versican in cultures of trisomy 7-carrying cells relates to an autocrine loop involving an increased IL-6 production.

Interstitial pressure gradients around joints; location of chief resistance to fluid drainage from the rabbit knee

The hypothesis has been advanced that synovium offers the main resistance to fluid escape from joints, even though it is under 20 microm thick. To test this, fluid was infused into the knee joint cavity of anaesthetised rabbits to set up a pressure gradient, then the profile of periarticular interstitial fluid pressure (P(if)) was measured by advancing a micropipette, connected to a servo-null pressure recorder, in steps through a periarticular tissue 'window' until the joint cavity was entered. With intra-articular pressure (P(j)) raised to 15 cmH(2)O (the pressure of an acute joint effusion) the pressure gradient dP(if) /dx (where x is distance) across the synovial lining was 0.47 +/- 0.04 cmH(2)O microm(-1) (n = 10 joints). This was 23.5-fold greater than the gradient in the subsynovium (0.02 +/- 0.01 cmH(2)O microm(-1); P < 0.0001, Student's t test), indicating that the hydraulic resistivity of the subsynovium is 4 % of that of the synovium. The pressure profile was not altered by circulatory arrest. To test the hypothesis further, the effect of a stab perforation of the synovial lining on fluid drainage rate ((.Q(s)) was studied. Perforation raised both.Q(s) and the conductance term d.Q(s)/dP(j) more than 10-fold (n = 6 joints; P < 0.0001, ANOVA). The results thus support the view that, despite its thinness, the synovial lining offers the main hydraulic resistance to fluid drainage from a synovial joint.

Ultracytochemistry of glycosaminoglycans in the canine knee synovium

The accurate localization and nature of glycosaminoglycans (GAGs) in the canine knee synovium were studied by ultracytochemical methods that involved high or low iron diamine-thiocarbohydrazide-silver proteinate (physical development) staining in combination with enzyme digestion control procedures. The results obtained indicated that heparan sulfates and hyaluronan were present mainly in the plasma membrane of the B (fibroblast-like) cells. In contrast, the plasma membrane of the A (macrophage-like) cells showed negative reactions after the histochemical examination. Dermatan sulfates, chondroitin sulfates (A and/or C) and hyaluronan were localized in the extracellular matrix of the synovial intima, whereby dermatan sulfates were confined to the fibrous component, whereas chondroitin sulfates and hyaluronan were found in the interfibrous matrix. Heparan sulfate was the only notable GAG molecular species localized in the basement membrane of the capillary wall. It is obvious that differences in the quality and localization of glycosaminoglycans in the canine synovial tissue are of specific interest in understanding normal functions as well as pathological alterations of the knee synovium in mammals.

Histochemical analyses of glycosaminoglycans in the synovial membrane of the canine knee joint

The accurate localization and nature of glycosaminoglycans in the synovial membrane of the canine knee joint were examined histochemically by means of the selective sensitized diamine procedures based upon high and low iron diamine stainings in combination with enzyme digestions. Using these methods, it was possible to clearly and easily detect exceedingly small amounts of glycosaminoglycans in synovial tissues, which cannot be visualized by methods employed to date. The sensitized high iron diamine (S-HID) procedure resulted in positive reactions of varying intensities in the intercellular matrix of synovial intima, and in the extracellular matrix and small capillary walls of the superficial layer in the synovial subintima, and also reacted vividly in the extracellular matrix and blood vessel walls of the deeper layer in the synovial subintima. In particular, the sensitized low iron diamine (S-LID) procedure resulted in positive reactions of the extracellular matrix in the synovial subintimal layers. The S-HID and S-LID procedures combined with the enzyme digestions proved that glycosaminoglycan molecular species such as chondroitin sulfate A/C, dermatan sulfate, heparan sulfate and hyaluronic acid are present in various concentrations in the synovial membrane of the canine knee joint. The present results were discussed with reference to the histophysiological and pathophysiological functions of glycosaminoglycans in the synovium of domestic mammals.

Effect of depletion of glycosaminoglycans and non-collagenous proteins on interstitial hydraulic permeability in rabbit synovium

1. The hydraulic resistance of synovial interstitium helps to retain a lubricating fluid within the joint cavity. The contributions of sulphated glycosaminoglycans to resistance were assessed by selective depletion by chondroitinase ABC, keratanase and heparinases I, II and III in vivo. Also, since glycosaminoglycans do not account fully for the resistance, the contribution of non-collagenous, structural proteins in interstitium was assessed by treatment with chymopapain, a collagen-sparing protease. 2. Ringer solution containing enzyme was injected into the synovial cavity of the knee in anaesthetized rabbits. After >= 30 min the intra-articular pressure was raised and the relation between pressure (Pj) and trans-synovial outflow (Qs) determined. The slope dQs/dPj at low pressures, i.e. below yield pressure, represents the hydraulic conductance of the lining, i.e. 1/resistance. The contralateral joint received Ringer solution without enzyme as a control. Action of enzymes on the tissue was confirmed by histochemical and immunohistochemical studies. 3. Treatment with chondroitinase ABC (5 joints) increased the hydraulic conductance of the lining by 2.3 times (control, 1.34 +/- 0.22 microliter l min-1 cmH2O-1; post-enzyme, 3.11 +/- 0.45 microliter l min-1 cmH2O-1). This was significantly less than the effects of leech, Streptomyces and testicular hyaluronidases, which caused an average 4.7 times increase (P < 0.001, ANOVA). Analogous findings were made above yield pressure. 4. Treatment with keratanase (3 joints) or heparinases I, II and III (3 joints) caused no significant increase in trans-synovial flows or conductance, even though the concentration of heparan sulphate in synovium is higher than that of chondroitin sulphates or hyaluronan. 5. Treatment with chymopapain (7 joints) caused the greatest increases in trans-synovial flow, which exceeded control flow by an order of magnitude in one case. After 0.1 U chymopapain the average conductance was 6.6 times the control conductance below yield pressure. Immunohistochemical studies confirmed that chymopapain treatment removed the synovial proteoglycans. 6. It is concluded that, despite their similar resistivities in vitro, the different glycosaminoglycans do not contribute equally, weight for weight, to interstitial resistance in vivo. Hyaluronan is the dominant glycosaminoglycan governing synovial interstitial resistance. In addition, non-collagenous structural proteins contribute significantly to interstitial resistance.

Effect of depletion of interstitial hyaluronan on hydraulic conductance in rabbit knee synovium

1. The hydraulic resistance of the synovial lining to fluid outflow from a joint cavity (Qs) is important for the retention of intra-articular lubricant. The resistance has been attributed in part to extracellular glycosaminoglycans, including hyaluronan and chondroitin sulphates. Increased permeability in joints infused with testicular hyaluronidase, which digests both chondroitin sulphates and hyaluronan, supports this view. In this study the importance of interstitial hyaluronan per se was assessed using leech and Streptomyces hyaluronidases, which degrade only hyaluronan. 2. Ringer solution was infused into the knee joint cavity of anaesthetized rabbits for 30 min, with or without hyaluronidase, after which intra-articular pressure (Pj) was raised and the relation between pressure and outflow determined. 3. Treatment with Streptomyces, leech or testicular hyaluronidases increased the fluid escape rates by similar factors, namely 4- to 6-fold. After Streptomyces hyaluronidase treatment the slope d 8d s/dPj, which at low pressures represents synovial hydraulic conductance, increased from a control of 0.90 +/- 0.20 microl min-1 cmH2O-1 (mean +/- s.e.m. , n = 6) to 4.52 +/- 0.70 microl min-1 cmH2O-1. The slope d 8d s/dPj increased to a similar level after testicular hyaluronidase, namely to 4.14 +/- 1.06 microl min-1 cmH2O-1 (control, 0.54 +/- 0.24 microl min-1 cmH2O-1). Streptomyces and leech hyaluronidases were as effective as testicular hyaluronidase (no statistically significant differences) despite differences in substrate specificity. 4. It was shown using histochemical and immunohistochemical techniques that hyaluronan was removed from the synovium by leech, Streptomyces and testicular hyaluronidases. The binding of antibodies 2-B-6 and 3-B-3 showed that the core proteins of the chondroitin sulphate proteoglycans remained intact after treatment with hyaluronidases, and the binding of 5-D-4 showed that keratan sulphate was unaffected. An azocasein digestion assay confirmed that the hyaluronidase preparations had no significant proteolytic activity. 5. The effect of the hyaluronidases was four times greater than predicted from the low concentration of interstitial hyaluronan and its resistivity. Factors that might amplify the effect of hyaluronan depletion include the matrix-organizing role of hyaluronan, and/or non-uniformity of hyaluronan distribution. It is concluded that interstitial hyaluronan makes a major contribution to synovial hydraulic resistance, but the mechanisms are as yet poorly understood.